Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
V WO 92/ 10431 ~ PCT/G B91 /02172
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IMPROVED WASTE WATER TREATMENT
Field of the Invention
This invention relates to an improved method of
treating waste water containing biodegradable matter and to
improved plant for implementing the method.
Discussion of Prior Art
It is known to break down environmentally damaging
matter in sewage or other waste water by means of bacteria
(e.g. naturally-occurring microbes) and to encourage the
growth of such bacteria and their digestion of the waste
matter thus creating biomass layers of media of an RBC
(Rotating Biological Contactor) which is supported partly in
air and partly in the waste water. The RBC is slowly
rotated to alternately immerse the biomass layers in the
waste water and then to expose them to atmospheric oxygen.
Normally the waste water is led to the RBC from an
upstream primary settlement tank and leaves the RBC to flow
into a final settlement tank from which, now purified of at
least the biodegradable material and suspended solids, it
flows on to the environment.
The usual procedure is to allow plant throughput to be
determined by the inflow of waste water and to dimension the
treatment plant so that even on the expected maximum rate of
input flow, an adequate residence time of the waste water in
contact with the RBC will be obtained. Such an arrangement
works well in most situations but in some extreme operating
conditions (e. g. where bioc.idal chemicals and petrochemical-
based cleaning agents may be periodically discharged in the
waste water) a steady decline in operating efficiency of the
plant has been noted which can only be resolved by putting
the plant out of commission and cleaning out a11 solid and
liquid material contained therein.
GB-A-2119357, EP-A-0161077 and US-A-4729828 a11
disclose waste water treatment apparatus in which treatment
is effected in at least two biological stages each
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containing a rotating biological contactor but there is no
suggestion in any of these documents that the upstream one
of these two stages forms a part of a flow-balancing chamber
for the apparatus.
By arranging for a biological contactor to form a part
of the flow-balancing chamber, the plant is able to cope
with waste water flows that at least occasionally contain
toxic substances that do inhibit the capacity of the biomass
to treat the biodegradable matter in the waste water.
Because a minor part of the biomass available on the RBC is
located in the flow-balancing chamber, this minor part can,
if occasion requires, act sacrificially to absorb, or at
least reduce, t.e toxicity and thereby prevent the flow of
toxic substances into contact with the major part of the
biomass on the RBC.
We have now found that by the simple expedient of
providing flow balancing at the upstream end of the RBC (and
optionally also by dividing the primary settlement tank into
two) it is possible to provide plug flow conditions in the
downstream part of the RBC and ensure that even if a
significant proportion of biocides is present in the input
flow, these can be effectively diluted in the primary
settlement tank thereby ensuring improved biodegradation on
the RBC, i.e. conditions approaching BOD removal kinetics
which are essentially first order with respect to BOD at the
resulting and applied mass BOD rates per unit area of RBC
media per day.
The invention is equally applicable to a compact
integrated sewage treatment plant designed to serve a single
house (e. g. a "Biodisc" (RTM) plant having an RBC media bank
housed in one tank with upstream primary and downstream
settlement tanks) up to a large-scale treatment plant
designed to serve a community of more than 1000 people where
a septic (primary settlement) tank is likely to be provided
separate from the RBC which in turn is separated from a
humus or final settlement tank.
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WO 92/l0431 PCT/GB91/021 c?
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Summarv of the invention
In its method aspect, the invention involves feeding a
chamber containing a rotating biological contactor (RBC)
with waste water from an inlet tank and discharging from the
RBC-containing chamber into a humus tank, the RBC being
divided into separated upstream and downstream sections, the
chamber containing the downstream section being fed with
water to be processed at a controlled rate from the chamber
containing the upstream section and a balancing chamber is
provided for isolating the downstream section of the RBC
from variations in flow of waste water to the inlet tank,
characterised in that the upstream section of the RBC forms
part of the balancing chamber.
Suitably the upstream section represents less than 50'a
Z5 of the total biomass support area of the RBC. Conveniently
the upstream section is between 20% and 40~0 of the total
biomass support area and suitably around 30ro.
Conveniently the inlet tank is divided into an upstream
section which receives the waste water and is maintained at
a constant water level and a downstream section which is
part of the balancing chamber and is hydraulically linked to
the chamber containing the upstream section of the RBC. In
this way the downstream section of the inlet tank and the
linked chamber containing the upstream section of the RBC
vary in liquid level as inlet flow fluctuates from hour to
hour and day to day while the upstream section of the inlet
tank and the chamber containing the downstream end of the
RBC remain at respective substantially constant liquid
levels.
The humus tank may include a sump into which solids
will settle and the method may include periodic removal of
the settled humus sludge to an upstream point in the waste
water treatment (e.g. to the primary settlement tank) or to
a point facilitating removal from the plant.
In its apparatus aspect, the invention comprises an RBC
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rotating in a chamber fed from an inlet tank and discharging
to a humus tank, the RBC chamber being divided into
separated upstream and downstream sections, and is
characterised in that the upstream section of the RBC
chamber forms part of a flow-balancing chamber for the
apparatus. Desirably the inlet tank is also divided into
upstream and downstream sections, the upstream section of
the RBC chamber and the downstream section of the inlet tank
together forming the balancing chamber.
A liquid lift means can be provided between the
sections of the RBC which is actuated by the rotational
motion of the RBC (e. g. by the provision of cups or the like
on the RBC) but an Archimedean screw, peristaltic pump, air
lift pump or electric pump could also be used.
Brief Description of the Drawings
The invention will now be further described, by way of
example with reference to the accompanying drawings, in
which .
Figure 1 is a schematic sectional side elevation of a
first form of plant operating in accordance with the method
of the invention, and
Figure 2 is a sectional side elevation of the invention
as applied to a Biodisc unit.
Description of Preferred Embodiment
Referring to Figure 1, a primary settlement tank 10 is
divided by a weir 11 into a first stage chamber 12 and a
second stage chamber 13. An RBC 15 is divided into an
upstream rotor section 15A constituting some 303'o of the
available biomass support areas and a downstream rotor
section 15B providing the remaining 700.
Sections 15A and 15B are rotatably supported in
separated RBC chambers 16 and 17, respectively. Both rotor
sections are driven by the same motor 18 and section 15A
r. .
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includes at least one bucket 19 on its downstream end to
lift a measured volume of liquid out of chamber 16 and to
discharge it into chamber 17 once each 360~ rotation of
rotor section 15A.
Chambers 16 and I3 are linked by plumbing 20 allowing
flow of liquid from chamber 13 to chamber 16.
Downstream of chamber 17 is a humus tank 21, the latter
being shaped to provide a sludge sump 21a from which an
output pipe 22 leads to a sludge accumulating tank 23 or to
an upstream part of the plant e.g. the tank 10.
An inlet for waste water into the plant is shown at 25
and an outlet for purified water is shown at 26. Under
operating conditions, the liquid levels throughout the plant
will be as shown by the dotted level. lines 30 to 34.
In chamber 12 the liquid level is constantly at level
30 set by the weir 11. In chamber 17 and tank 21 the liquid
levels are constant respectively at 31 and 32 and are set by
the level of an outlet 27 from the chamber 17 and the pipe
26 (or outlet weir) from tank 21. In chambers 13 and 16,
however, the liquid level can vary anywhere between lines 33
and 34 due to the fact that flow into chamber 13 is related
to the rate at which inlet waste water flows into chamber
12, whereas the outflow rate from chamber 16 is constant
provided the RBC section 15A is rotating at a constant rate
and providing the level in chamber 16 is not close to the
lower level limit 34. Levels 30, 33, 31 and 32 could be the
same, but need not be.
Controlling the levels in the manner described has a
dramatic effect on the performance of the plant particularly
so in the presence of periodic discharges of biocidal and
petrochemical-based materials in the waste water entering
the chamber 12. It is felt that this surprising improvement
in long-term efficie:'y derives from the fact that chambers
12 and 13 serve as a septic tank and the dwell time o: fresh
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effluent in the tank 10 under conditions of low redox
oxidation potential (ROP) will reduce the toxic effect of
many biocides so that when they are released from chamber 13
they have become wholly or partly degraded and diluted to
the point where their toxic effect is markedly reduced there
by reducing the inhibition of the biodegradation process of
the RBC in the upstream chamber 16 and certainly not damage
the biomass on the RBC section 15B.
The drawing shows (in chain Lines) a link 35 between a
bottom part of the chamber 16 and the chamber 12 and this
can be useful for passing sludge which drops off the rotor
section I5A back into the tank 10 e.g. via the inlet 25.
The upstream section 15A of the RBC which is housed in
the chamber I6 forms the upstream part of a biozone and
contributes to the process in the following manner:
( a ) The inhibitory effect of any toxic chemicals in t.~-:e
waste water will be substantially reduced or eliminated in
the upstream section of the RBC. The chemicals would
undergo primary biological degradation and any detrimental
effuect on the biochemistry will be reduced or buffered.
This will enable the downstream section of the RBC to
perform consistently without inhibition.
( b ) The upstream section of the RBC will improve the
Redox Oxidation Potential (ROP) by reducing or overcoming
septicity of the liquid emanating from the septic tank 10.
(c) The downstream chamber 17 will receive waste water
from the upstream chamber 16 at a controlled rate. The
fixed film process of BOD removal on an RBC is first order
with respect to BOD at low organic mass loading rates and
therefore the downstream section RBC's efficiency is
improved. Also, the velocity constants of BOD removal (BOD
due to some chemicals present in waste water) are small,
therefore high retention times achieved on the downstream
RBC section due to balancing upstream, ensures a high degree
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WO 92/10431 ~ ~~ ~ ~. ~ PCT/GB91/021'2
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of consistent BOD removal.
(d) The fixed film process of ammoniacal nitrogen
removal in the RBC is between zero and half order witY~
respect to ammoniacal nitrogen at the low mass loading
rates, and also the velocity constants are very small. The
downstream RBC section 15B is provided with stable condi
tions (with respect to flow rate and pollution load) for the
process to operate as near the ideal conditions as possible
and thereby enabling a high degree of treatment
consistently.
(e) By removing and returning sludge from the upstream
chamber 16 the BOD load, due to solids, otherwise applied to
the downstream RBC section, is reduced. This therefore
improves the overall process efficiency.
The following example further illustrates the inven-
tion.
A farm-based RBC conventional treatment plant which
received waste water from a domestic dwelling 36 (average
flow 1000 litres/day) and a milking parlour 37 (average flow
100 - 150 litres/day) was found to deteriorate in
performance over a ten week cycle. At start-up, the BOD
(biological oxygen demand) of the treated water discharged
from the plant outlet 26 was 30 ppm (30 mg/litre) but this
gradually increased to lOQ.ppm over a period of a few weeks.
Cleaning out the plant and restarting saw a return to the
acceptable initial <30 ppm figure but again a steady
deterioration of performance set in over the following weeks
requiring yet another clean-out of the plant when the
efficiency again became unacceptable. It is suspected that
chemicals in the waste water from the milking parlour 37
(e. g. to name one M.I. acid) acted as a biocide.
On converting the plant to operate in accordance with
the method of this invention, the waste water from the
milking parlour 37 and the domestic sewage from the dwelling
WO 92/10431 ~ ~ ~ ~ ~ ~,.~ PCT/GB91/02l72
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36 were both discharged into the first stage 12 of the
septic tank 10. The clarified mixed liquid then discharged
over the weir 11 into the second stage 13 of the septic tank
which now acts as part of a balancing chamber.
The first stage of the RBC is separated from the second
stage by a fixed baffle 38. The second chamber 13 of the
septic tank being linked with the first stage chamber 16 of
the RBC hydraulically at a lower submerged level via the
plumbing 20, acts together as the balancing chamber for the
plant. The balanced flow is transferred from the first
stage RBC chamber 26 to the second stage RBC chamber using
the buckets) 19. In the example given, this rate would be
of the order of a forward feed rate of 1.2 litres/min..
The first section 15A of the RBC acts to condition the
sewage and will overcome any partial inhibition which may
occur because of biocides persisting in the flow. The
balancing in chambers 13 and 16 will enable the second
section 15B of the RBC to operate in a plug flow mode at the
desired mass BOD loading rate (in the example given a rate
of approximately 3 gms/m2/day). The controlled flow
throughput in chamber 17 increases retention time in the RBC
thus taking care of the low velocity constant and also
enables the humus tank 21 to operate at an almost constant
flow. The mixing of the domestic sewage and milking parlour
waste water in chamber 12 ensures more uniform liquid
composition and adequate dilution levels for the washing
liquids from parlour 37 thus reducing the organic and
chemical shock loads on the RBC.
To ensure good mixing of input water upstream of the
weir 11, a baffle 39 can be provided adjacent to the inlet
25.
The RBC speed should be set (e. g. at 2 rpm) to avoid
excess oxygen in the bulk~liquid in chamber 17. This is to
create oxygen-limiting conditions in the bulk liquor and
provide oxidation of the pollutants in the biofilm on the
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rotor section 15B rather than the creation of mixed liquid
suspended growths.
The humus tank 21 serves as a proper settling tank
which facilitates consolidation of sludge and its effective
removal via the pipe 22. To avoid disturbing the settled
sludge, a desludging operation can be carried out by timer
controlled, pump-assisted hydrostatic means (not shown).
The sludge recovered from the pipe 22 could be removed from
the plant at intervals but could be returned to the septic
tank 10 if desired.
With the adaptation described, the plant receiving the
output from 36 and 37 operated in steady state conditions
with less than 30 ppm BOD in the discharged liquid.
It is our knowledge that naturally occurring microor-
ganisms can eventually adapt to even quite hostile environ-
ments if the environment is not constantly varying. The
balancing of the flows into the RBC chamber 16 appears to
enable this to be achieved. If naturally-occurring micro-
organisms have problems adapting to the environment,
cultured adapted microorganisms with inbuilt resistance to
the chemicals in the wash water can be used to inoculate the
plant. However inoculation of the plant with cultured
microorganisms should only rarely be necessary.
Although Figure 1 shows a plant with separated primary
tank Z0, chambers I6, 17 and humus tank 21, the invention
can equally well be applied to integrated plant (such as
that known under our Registered Trade mark "BIODISC") in
which a single tank is sectioned to provide interconnected
volumes 10, 16, 17 and 21 and such a tank is shown in Figure
2. In this plant a slot 40 is provided in chamber 16 to
allow sludge dropping off the rotor section 15A to pass back
into the volume 10.
